1. Field of the Invention:
The invention relates to power line carrier apparatus used in protective relaying systems for the protection of electrical power systems.
2. Description of the Prior Art:
Certain types of protective relaying systems for monitoring and protecting electrical power systems utilize power line carrier of the on-off type for communication between local and remote stations. Predetermined system parameters are monitored at each station, and signals responsive to the monitored parameters are sent to the other station via power line carrier for comparison in a protective relay. The comparisons indicate whether a detected fault is internal, i.e., between the two stations, in which event the line sections should be isolated by tripping associated breakers, or external, in which event the circuit breakers are not tripped. For example, in a phase comparison protective relaying system, the three-phase line currents at each station are converted to a single-phase voltage. If there is no fault between the stations, these voltages will key their associated transmitters on alternate half cycles of the power system frequency. An internal fault will cause the stations to key their transmitters on the same half cycle. Thus, it is important that the receivers at each station receive and recognize a signal from the transmitter at the other station without delay, as an inaccurate representation of the phase relationship may cause improper operation of the protective relay. U.S. Pat. No. 2,406,615 entitled "Carrier Current Relying", is an example of a protective relaying system which utilizes on-off carrier for communication.
While in some carrier-current installations, the transmitters transmit at different radio frequencies, in others they transmit at the same radio frequency. When the transmitters operate at the same frequency, or a frequency within the pass-band of the input filter of the associate receiver, the local receiver is forced to operate whenever its local transmitter is transmitting a carrier signal. Under typical operating conditions, the receiver's sensitivity is set to respond to a signal level which is considerably lower in magnitude than that of the local transmitter output, to which it is directly connected. This forces the receiver circuitry to operate with an overwhelming carrier input signal level whenever the local transmitter is operating. This condition can cause a considerable time delay in the receiver drop-out response to the local transmitted carrier when it stops. This delay is primarily caused by the receiver input filter having to dissipate the relatively large carrier energy provided by the local transmitter, causing it to oscillate or "ring", and produce an output for a period of time after the carrier transmission has stopped. This condition has its most serious detrimental effect when the local and remote transmitters are keyed on and off alternately at the 60 Hertz power frequency rate, as would be the case when the hereinbefore mentioned "phase-comparison" relaying is employed. Thus, the critical timing relationship between the pulses derived from the receiver response to the local transmitter, and the receiver response to the remote signal, may be disrupted.
Thus, it would be desirable to be able to prevent overload of the band-pass input filter of the local receiver by the signal from the local transmitter, without resorting to costly squelch circuits following the input filter, or costly R-F hybrids which deleteriously affect either the transmitter output power efficiency, or the receiver sensitivity.